1. Field of the Invention
The present invention relates to workholding apparatuses and, more particularly, is directed to a method and apparatus for positioning a stationary clamping element on a precision machine vise within a predetermined range of accuracy.
2. Description of the Invention Background
There are a number of known methods for accurately positioning a member on a planar surface of an object. For example, FIGS. 1 and 2 illustrate a known method for positioning a member 212 on an object 214 having a planar surface 216. As shown in FIGS. 1 and 2, two spaced-apart, round pins 218 are attached to the member 212 such that they extend therefrom to be received in corresponding round holes 220 provided in the object 214. To ensure that the pins 218 can be inserted into the holes 220, the distance "A" between the centers of the pins 218 and the distance "B" between the centers of the holes 220 must be precisely controlled so that those distances are substantially equal. Such precision machining is costly and time consuming. As such, to accommodate slight variations between those distances, which can result in misalignments between the pins 218 and the holes 220, the holes 220 are typically fabricated with larger diameters than the outside diameters of the pins 218. However, that action can lead to positioning inaccuracies depending upon the magnitude of the clearance provided between the pins 218 and their corresponding holes 220.
Another known positioning method for positioning an object 312 on a member 314 having a planar surface 316 is illustrated in FIGS. 3 and 4. When using that method, two round holes 320 are provided in the object 314. A round pin 318 and a diamond shaped pin 322 are attached to the pin 312 such that they extend therefrom. The diamond shaped pin 322 typically consists of a round pin with tapered sides 324 machined thereon such that when viewed from one end, the pin appears to be diamond shaped. Slight variations between the distance "C" between the centers of pin 318 and diamond shaped pin 322 and the distance "D" between the centers of the round holes 320 is largely accommodated by the tapered sides 324 of the diamond shaped pin 322. As such, less clearance is needed between the pin 318 and its corresponding hole 320 when compared to the first prior method discussed above. To ensure accurate positioning of the member 312 on the planar surface 316, however, the diamond pin 322 must be rigidly and non-rotatably attached to the member 312. As such, the diamond pin 322 cannot be threadably attached to the pin 312 and, accordingly, other more time consuming fastening methods must be used to fasten the diamond shaped pin 322 to the member 312.
The above-mentioned fastening methods are typically used to obtain acceptable positioning accuracy when affixing stationary clamping elements onto the base member of a precision workholding device. Precision workholding devices are typically used in connection with various machine tools and serve to hold stock pieces of material or workpieces in a precise location during the machining operation.
There are a variety of known apparatuses for holding a workpiece during machining operations. Such apparatuses may comprise apparatuses ranging from simple "C"-clamp operated devices to precision machine vises that have clamping members adapted to be very precisely located with respect to a fixed reference location.
Many workholding apparatuses, such as the one disclosed in U.S. Pat. No. 5,022,636, are adapted to clamp a workpiece between opposing jaw members on a base. Typically, one jaw member is fixed and the other jaw member is movable relative thereto via a screw member. As the movable jaw member is moved towards the fixed jaw member, the workpiece is clamped therebetween. With those devices, however, the members used to position the movable clamping elements of precision workholding apparatuses are typically exposed to chips and debris created by the machining process. Therefore, in order to insure that the chips and debris will not impair the operation of the workholding apparatus, the apparatus must be carefully cleaned after each machining operation or protective covers must be installed. One type of protective cover normally used to prevent chips and debris from hampering the positioning member's operation is a flexible bellows-like cover that is typically attached to the movable clamping element and the fixed clamping element and is arranged to extend therebetween. Those covers, however, because they must extend and collapse in response to the position of the movable clamping element, may sometimes become jammed in the positioning mechanism. In addition, the debris and chips from the machining operation can also collect in the folds of the bellows making the eventual cleanup operation more difficult and time consuming. Further, bellows-like covers afford little protection to the inner-workings and the base of the workholding apparatus should it be accidently dropped.
In addition, because the clamping elements are susceptible to being accidentally advanced out of the base member, screw stops are typically employed to restrict the travel of the clamping elements. Those screw stops, however, must be removed each time the workholding device is cleaned and, thus, increase the amount of time required to clean the workholding apparatus.
In view of the foregoing, there is a need for a method and apparatus that afford acceptable positioning accuracies when positioning a member on a plane surface. There is another need for an apparatus that can be used in connection with precision workholding devices that can limit the longitudinal travel of the clamping elements without hindering the cleaning process. There is a further need for an apparatus that can protect the inner workings and the base member of a workholding apparatus from the chips and debris created during the machining process without hampering the operation of the apparatus.